1. Field of the Invention
This invention lies in the field of energy recovery. More particularly, it is concerned with the in situ combustion in shallow thin beds of coal, at shallow depths below the surface of the earth. Still more particularly, the invention lies in the field of the combustion of the coal by supplying air to the coal seam from the surface and utilizing the heat generated by the coal combustion to generate steam which is utilized at the surface of the ground, utilizing the earth (coal and surrounding rocks) as the furnace in the same manner as if it were geothermal.
In one application, this invention lies in the field of outcropping seams of coal where horizontal conduits can be drilled and placed within the coal seam to a specific point, where they join a vertical pipe. Water supplied to the horizontal pipes at the outcrop is heated within the pipes by the combustion of the coal around the pipes, and steam thus formed is carried up the vertical pipe to be used to drive a turbine generator system.
A still further application lies in the field of utilizing coal seams thin or thick, shallow or deep overlain by water sands or by hydrocarbon bearing formations whereby fires are started within the coal seam, and the heat of combustion is then communicated to the overlying aquifer or oil formation. If the overlying bed is an aquifer then steam will be formed which can be carried to the surface to drive an electric generator system. If the overlying bed contains viscous oil, then the heat will cause the oil to be reduced in viscosity sufficiently so that it can be driven by a steam drive, or by pumping water into the seam, which in view of the fire in the coal seam will turn to steam, which will drive the oil to the surface to be collected.
Still a further application involves the burning of coal in situ and carrying the hot gaseous products of combustion up a vertical borehole past a coiled pipe carrying water. Heat is transferred to the water by convection, forming steam which is carried to a turbine, etc.
2. Description of the Prior Art
Subsurface combustion of coal has been carried on in nature, by fires of unknown origin which were started many years ago and are burning continuously to the present date. Also oil companies, coal companies and government research laboratories have investigated many ways of underground combustion of coal, so as to utilize the products of combustion directly, as a coal gasification system. However, none of these methods have become practical as of the present date because of one or more basic difficulties.